Polysilicon thin films of improved electrical uniformity

ABSTRACT

A method for forming a polysilicon thin film semiconductor device precursor, and the precursor, are disclosed, wherein the deposited thin film layer is scanned with a continuous wave laser in a first direction, and scanned a second time in a direction different from that of the first direction. The cross-scanning reduces the anisotropy of the thin film produced by the first scanning and apparently induces larger grain size in the recrystallized polysilicon.

This is a divisional of application Ser. No. 435,221, filed Oct. 19,1982, now U.S. Pat. No. 4,466,179.

FIELD OF THE INVENTION

This invention relates to the art of fabricating silicon integratedcircuit precursors and associated devices, and specifically, to the useof a continuous wave laser for annealing thin films of polysilicon insuch devices.

BACKGROUND OF THE INVENTION

The use of lasers to melt and recrystallize polysilicon thin films invarious silicon semiconducting devices has received increased attentionin the semiconductor industry over the past few years. Polysilicon, asdeposited using low pressure chemical vapor deposition (LPCVD) or othertechniques, has serious drawbacks which limit its use for thin filmresistors or active devices such as transistors. The main problem withthe deposited film is that it generally consists of many small, randomlyaligned crystallites separated by small grain boundries which can bepractically considered as defects in the thin film. The grain boundriesact as trapping centers, preventing polysilicon films from being used inactive areas of devices, such as the base of a bipolar transistor, dueto the extremely low minority carrier life time in the polysilicon. Thegrain boundaries are also the cause of a conductive mechanism formajority carriers which is extremely temperature-sensitive, resulting inhigh negative temperature coefficients when these films are used in thinfilm resistors, substantially detracting from their value.

One approach to eliminating these problems has been to melt thepolysilicon and allow it to recrystallize in larger grains, thusreducing the density of the above-described boundries and improving theelectrical characteristics of the film. One method of melting andrecrystallizing which has received substantial attention throughout theindustry is the use of laser irradiation, particularly continuous wavelasers. Unfortunately, it has been discovered that this technologysuffers from its own drawbacks.

One problem encountered in using a scanned continuous wave (CW) laserfor melting and recrystallization of polysilicon films is that thistechnique generally induces severe electrical anisotropy in the film.The conventional technique for scanning the laser beam across thesemiconductor wafer is believed responsible for this effect. The laserbeam is usually scanned in one direction across the wafer. Since thewidth of the laser beam is very small, the beam is scanned many times,each time stepping the beam a small distance in a directionperpendicular to the scan, in order to cover the entire wafer. Thisresults in many parallel, closely spaced lines of recrystallizedpolysilicon grains on the wafer. When a CW laser is used, thepolysilicon grains recrytallize in long, thin crystallites in parallelarray along the scan lines. It is this structure that is believedresponsible for the severe electrical anisotropy found in such films.Electricity flows much more easily along the parallel lines ofcrystallites generated by the scan line than across them, because thedensity of the grain boundries across the scan lines is much higher thanthat along the scan line. The electrical anisotropy induced is areflection of the spatial anisotropy of the grains generated by parallellaser scan lines.

Accordingly, it is one object of this invention to provide a process forthe melting and recrystallization of polysilicon films that does notinduce electrical anisotropy into the film.

A second object of this invention is to provide a process of melting andrecrystallization of polysilicon that increases the grain size of therecrystallized polysilicon over that obtainable by conventionalprocesses, thereby reducing the sheet resistance of the treated film.

Other objects of the invention will be apparent from its description setforth below.

SUMMARY OF THE INVENTION

The severe electrical anisotropy introduced in polysilicon thin filmswhich have been melted and recrystallized through the use of a CW lasercan be overcome by scanning the film a second time in a direction at anangle to that of the first scan. This second scan causes therecrystallized grains to orient themselves in a direction other thanthat parallel to the original scan lines, and also appears to increasethe grain size of the recrystallized polysilicon in the semiconductordevice precursor so formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the semiconductor deviceprepared to demonstrate the process of the invention.

FIG. 2 is a representative drawing of the apparatus employed to measurethe resistance of the product of the process of this invention.

DETAILED DESCRIPTION OF THE INVENTION

It has been discovered that the electrical anisotropy introduced intopolysilicon thin films by scanning in one direction with a CW laser canbe overcome by scanning in a second direction, or "cross-scanning". Thiscross-scanning tends to eliminate the spatial anisotropy introduced bythe original scan, thereby removing the electrical anisotropy discussedabove.

The polysilicon film is first scanned with the continuous wave laser inconventional fashion, i.e., in narrow, closely spaced parallel linesacross the entire wafer. The once-scanned polysilicon film is thenscanned again, using a CW laser. The second scan direction is at anangle to that of the first scan, the bias being introduced most easilyby rotation of the wafer after the first scan is completed.

The melting and recrystallization caused by the second scan allows forrecrystallized grains to be formed in such a fashion that they are notexclusively oriented along parallel scan lines, either those of thefirst or second scanning. Although it will be apparent that any angle ofbias between the first and second scanning will avoid uniform parallelorientation of the grains, a rotation of the wafer by 90°, so that thesecond set of scan lines is perpendicular to the first, is a preferredembodiment. In this embodiment, the electrical anisotropy of the film,as illustrated by the difference in resistance across the film inorthogonal directions, can be reduced below 2%.

It has been discovered that not only does this technique result in alarge reduction in the electrical anisotropy observed after a singlescanning, and the uniformity of the sheet resistance throughout the filmafter subsequent processing to form thin film resistors being alsoimproved; but it is also believed the same cross-scanning techniquegenerates grains of increased size when compared with single scanningtechniques, contributing to a reduction in the sheet resistance of thefilm.

The advantages of the process of this invention can be furtherunderstood with reference to the specific examples set forth below.

EXAMPLE 1

In order to compare the performance of polysilicon thin films irradiatedaccording to the process of this invention with those of prior artprocesses, the following experiment was performed, illustrated in FIG.1.

A first lot 1 of wafers was produced, starting with substrates 1 of Ntype,3-5 ohm-cm, <1-0-0> silicon. It should be understood that this typeof substrate is example only, the process being susceptible inapplication to a wide variety of substrates. These substrates wereoxidized to grow a thermal oxide layer 2 of about 1,000 Å. 3,000 Å ofLPCVD polysilicon 3 was then deposited on the oxide. The wafers werethen irradiated with a CW argon laser to melt and recrystallize thepolysilicon film. The wafers were then rotated 90° and scanned in adirection perpendicular to the first scanning, or cross-scanned. Thewafers were then ion implanted with phosphorous at a potential of 65KeV. The wafers received a dose of 5E13 ions/cm². A thin film resistorpattern was then delineated on the polysilicon film, the wafers wereoxidized, and 2,000 Å of silicon nitride 4 was deposited to act as apassivation layer for the resistors.

Contact apertures 5 were etched through the nitride layer and aphosphorous deposition was performed to allow ohmic contact to be madeto the ends of the resistors. Interconnect metal depositions 6 anddelineations were then performed to allow electrical probing of theresistors. As shown in FIG. 2, in order to determine the degree ofanisotropy in the films after processing, the resistance betweenorthogonal opposite pairs A and B and C and D of apertures on each edgeof the annealed film was measured. For isotropic films, the tworesistance values should be equal. The greater the difference in the tworesistances, the greater the anisotropy. The average of the wafers oflot 1 is given in Table 1.

COMPARATIVE EXAMPLE 1

In order to duplicate the recrystallized polysilicon films ofconventional techniques, and compare them with the performance of thewafers prepared according to this invention, a second lot 2 of waferswas prepared in identical fashion to the wafers of lot 1. However, thewafers of lot 2 were scanned only in one direction, no scross-scanningoccuring. In contrast to the cross-scanned wafers, each of thesingle-scanned wafers of comparative Example 1 received a dose of 1E14ions/cm².

The wafers were thereafter processed in a fashion identical to thewafers of lot 1, and identical measurements taken.

As can be seen from the comparison in Table 1, the cross-scanningtechnique of this invention has reduced the anisotropy introduced byconventional processes from a level which would be consideredunacceptable for integrated circuit use to a level which is tolerable.Table 1 also suggests that the cross-scanning has increased the grainsize over uni-directional scanning, as evidenced by the lower sheetresistance, in spite of the fact that the cross-scanned wafers wereimplanted with only one-half the dose of phosphorous of theuni-directionally scanned wafers.

                                      TABLE 1                                     __________________________________________________________________________                     Resistance Across First                                                                   Resistance Across Second                                Scanning Technique                                                                      Set of Paired Opposite                                                                    Set of Paired Opposite                                  Employed  Aperture ohms/□                                                                Aperture ohms/□                                                                 % Difference                        __________________________________________________________________________    Example 1                                                                            Cross-Scanning                                                                          637.9       629.9        1.37                                Comparative                                                                          Uni-Directional                                                                         894.3       838.1        6.37                                Example                                                                       __________________________________________________________________________

The invention has been described with respect to particular embodimentsthereof. It will be apparent that many variations within the scope ofthis invention will occur to those of skill in the art. Particularlyvariations in the angle of scanning direction and particular type oflaser employed will be apparent without the exercise of inventivefacility.

What is claimed is:
 1. A polysilicon thin film semiconductor deviceprecursor of reduced anisotropy, comprising:a substrate with apolysilicon thin film deposited thereover which film has been scanned afirst time by a laser beam in a first direction to produce relativelylong, thin crystallites having a long axis oriented generally parallelto the direction of said first scan, and scanned a second time by alaser beam in a direction at a bias to said first direction, whereby therecrystallized crystallites of the polysilicon thin film are caused togrow in the direction of said second scanning.
 2. The polysilicon thinfilm semiconductor device precursor of claim 1, wherein sad secondscanning of said polysilicon film is at an angle of 90° to the directionof said first scanning.
 3. The polysilicon thin film semiconductordevice precursor of claim 1, wherein the resistance across saidpolysilicon thin film in a first direction is substantially equal to theresistance of the polysilicon thin film in the direction at right angleto the direction of said first resistance.
 4. The polysilicon thin filmsemiconductor device precursor of claim 3, wherein the differencebetween said first and said second resistances is no greater than 2%. 5.A polysilicon thin film semiconductor device precursor of reducedanisotropy, produced by a process which includes scanning the precursorwith a laser beam in a first direction, followed by scanning theprocursor a second time in a second direction at a bias to said firstdirection, comprising:a substrate with a polysilicon thin film depositedthereover, said film comprising recrystallized crystallites having afirst angle of orientation generally aligned in the direction of saidfirst scan, and having a second angle of orientation generally alignedin the direction of said second scan.